The present invention relates to heat treatment of shaped articles, larger than films or fibers, formed from anisotropic-melt-forming polymers, especially polyesters and copolyesters.
Annealing of molded plastic articles to relieve stresses is well known in the art. This type of heat treatment does not generally improve mechanical properties, particularly toughness. Upon annealing prior art polyesters, for example, polyethylene terephthalate, elongation and toughness generally decrease even though temperatures are used which are quite low compared to the melting point of the polymer. Typical results are described in Journal of Macromolecular Science, Physics B 8 (1-2) pages 343-359 (1973) by R. M. Mininni, R. S. Moore, J. R. Flick, and S. E. B. Petrie. This publication discloses that annealing of a film at a temperature of 51.degree. C. for 90 minutes reduces elongation by more than 70%. Although the time scale is longer, other work shows similar effects for more massive shapes.
In view of these well-known effects, other than for removal of stress heat treating is not generally employed for improving mechanical properties of a polymer.